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The effect of pressure on carbon-blackelastomer powders. Part II Modulus and swelling measurements on HAF-blackSBR-powders at pressures up to 19 kbar

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D i r Anqcw.andr~*Mu&romolt.kularr Chrmir 36 f I Y 7 4 ) I - 16 ( N r . 4 9 7 )
From the Dunlop-Forschungslaboratorium. 645 Hanau. Germany
The Effect of Pressure
on Carbon-Black/Elastomer Powders
Part II: Modulus and Swelling Measurements on HAF-BIackjSBR-Powders at Pressures
up to 19 kbar
By Helmut Schilling, Gerd Angerer. and Tet Soei Ng
(Received 12 June 1973)
Powders from HAF-black/SBR mixtures in the total range of composition from 0
to 100% SBR were prepared by spray-drying mixtures of aqueous dispersions of HAFblack and SBR.
Application of static pressure up to 19 kbar to these HAF-blackiSBR powders at room
temperature yielded coherent specimens which i n many respects behaved likc vulcanized
HAFISBR compounds.
The modulus-pressure, modulus-temperature. swelling and solubility bchaviour of these
systems was studied. The torsional modulus which was measured betwecn - 100 and
+ I50 C showed an increase over several decades with increasing carbon-black content.
From the equilibrium swelling in toluene the apparent crosslink density of the polymer
in the pressed HAFiSBR specimen was estimated. I t was found that the apparent crosslink
density increases with increasing carbon-black content and also with the pressure at
which the samples were prepared.
Theapplication ofVergnon's theory about the pressure dependence of the bulk modulus
of metal powders to HAF-black permitted to calculate the density of HAF-black at
ideal packing conditions as 2.00 g/cm3~
Durch Spruhtrocknung von Mischungen waOriger Dispersionen von hochabriebfestem
OlruB (HAF) und Butadien-Styrol-Kautschuklatex (SBR) wurden Pulvermischungcn im
gesamten Mischungsbereich von 0 bis 100% SBR hergestellt.
Durch Einwirkung von statischem Druck bis zu 19 kbar auf diese HAF/SBR-Pulver
bildeten sich bei Raumtemperatur kompakte Prufkorper. die sich in manchen Eigenschaftcn wie HAF/SBR-Vulkanisate verhielten.
Das ModuVDruck-. Modulnemperatur-. Quellungs- und Loslichkeitsverhalten dieser
Systeme wurde untersucht. Der Torsionsmodul. der zwischen - 100 und + I50 C gemcssen wurde. zeipte einen Anstieg iiber mehrere Zehnerpotenzen mit steipendem RuOgchalt.
Die scheinbare Vernetzungsdichte des Polymeren in dem geprenten HAFISBR Prufkorper
H. Schilling, G. Angerer, and T. S. Ng
wurde aus der Gleichgewichtsquellung in Toluol ermittelt. Es wurde gefunden, daD
die schcinbare Vernetzungsdichtc sowohl mit stcigendem RuBgchalt als auch mit steigendem Herstellungsdruck der Proben ansteigt.
Die Anwendung einer Theorie von Vergnon uber die Druckabhgngigkeit des Kompressionsmoduls von Metallpulvern auf HAF-RulJ gestattete die Berechnung der Dichte
des HAF-Rubes unter optimalen Packungsbedingungen zu 2,OO g/cm3.
In part I of this work' density measurements were described which were
carried out at pressures up to 19 kbar on HAF/SBR powders of different
composition. In the present paper studies of bulk modulus at pressures up
to 19 kbar, temperature dependence of the torsional modulus, swelling behaviour, apparent crosslink density, solubility, and repeated pressure treatment
will follow which will add further information to the results already obtained.
Carbon-black/elastomer powders were prepared by spray-drying mixtures of aqueous
dispersions of high abrasion furnace (HAF) black with styrene-butadiene-rubber(SBR)
latex. The carbon-black/polymer ratio was varied in the range IOO/O, 95/5, 75/25, 67/33,
50/50, 33/67 and 0/100 parts by weight. The method of preparation of the powders
was the same as specified in Part 1'. Coherent samples were obtained by pressure
treatment of the powders in the already described cylindrical ccll and also in a cell
with a cavity of I M m m x 6mm x 1 mm for the preparation of strip-like samples. This
cell was built in the workshop of our laboratory.
The press in which this ccll was inserted was a 120 ton press (400mm x 400mm
curing press from Troester) with additional pressure distribution platens. Maxihum
pressure on the samples was 20 kbar.
Bulk Modulus at Pressures up to 19 kbar
The bulk modulus of solids and liquids can be determined experimentally
from compressibility measurements. In the present study we have to deal
with three types of material which behave differently when compressed:
1. Polymer without filler (SBR),
2. The composite system polymer/filler (SBR/HAF-black),
3. Filler without polymer (HAF-black).
Modulus and Swelling Mea.surc.ments
In the past many investigators have studied the compressional behaviour
of polymers without filler; Bridgeman' and Weir3 carried out their investigations at pressures above 2 kbar whereas Hellwege et aL4, Wood and Martin5,
and Grindstaff and Griskey6 did the same in the pressure range below
2 kbar. Recently Heydemann and Houck- measured bulk modulus and density
of polyethylene at pressures up to 30 kbar.
Only few investigators studied the compressional behaviour of composite
systems of polymers and fillers. Van der Wal et a1.' investigated the model
system polyurethane rubber/sodium chloride. Surland' investigated the compressional behaviour of elastomers which contained crystalline fillers and
microvoids as well. The study by Yurchenko et a1.I' about the compressional
behaviour of the system asbestos/rubber was already mentioned in Part I
of this work. All three authors carried out their measurements in pressure
ranges below 1 kbar.
More work was done in fields which are related to the compressional
behaviour of fillers without polymer. Here the preparation of tablets in pharmacy and of sinter-metal in metallurgy is an often studied problem. Some
of the results of these investigations can well be applied to the compression
ofcarbon-black. This is especially true for the work of Unckel'', Jovanovic",
and Vergnon ' 3.
From our own density-measurements described in the preceding publication'
we can calculate the bulk moduli of the different powders in the pressure
range between 0 and 19 kbar. For the determination of the bulk modulus
of styrene-butadiene-rubber (SBR) without filler an experimental procedure
as described by Heydemann and Houck' is appropriate, especially because
the geometrical dimensions of their samples were very similar to ours. (A
repetition of their experiments on polyethylene in our apparatus gave almost
identical results.)
The bulk modulus B = -Vc, .dp/dV is equal to - L,;dp/dL where V, volume
of the sample at zero pressure, V volume of the sample at pressure p, Lo
height of the sample in the pressure cell at zero pressure, L height of the
sample at pressure p. Lo and L were measured as described earlier and
from these values B was calculated for different pressure intervals.
Fig. 1 shows the experimentally determined density of SBR-powder at
different pressures and Fig. 2 shows the corresponding bulk modulus
- V, .dp/dV derived therefrom. The relation between bulk modulus and pressure is linear with exception ofthe region near the origin. This deviation is caused
by the compressional behaviour of the powder which at low pressure is
H. Schilling, G. Angerer, and T. S. Ng
D e n s i t y ( g.cmi3)
Fig. 1 .
Pressure (kbar)
Pressure dependencc of the density of SBR-powder.
Bulk modulus (kbar)
- powder
Fig. 2. Pressure dependence of the bulk modulus of SBR-powder.
Modulus and Swelling Measiirements
a soft material and with increasing,pressure is gradually compacted until
it behaves like a bulk material. Apart from this the results plotted in Fig.
2 are very similar to bulk modulus/pressure measurements carried out by
other workers on various other elastomers, e.g. Bridgeman2 on butyl rubber
and natural rubber, WarfieldI4 on polysiloxane and Adams and Gibsonls
on natural rubber.
Wood and Martin5 in connection with their measurements on natural
rubber up to 0,5 kbar pointed out the applicability of the Tait-equationI6
to describe the compressional behaviour of elastomers:
where b and c are constants.
They also showed the applicability of this equation to similar measurements
on various elastomers carried out by other authors at pressures up to 25
By differentiation of equation (1) the compressibility K is obtained
and from this follows the bulk modulus B= 1/K.
The constants b and c can be determined from the bulk modulus/pressure
curve when the latter is truly linear. In our case due to the deviation from
linearity at low pressures only c was determined from the slope of the curve
plotted in Fig. 2 whereas the constant b was determined from the density/pressure curve plotted in Fig. 1.
For the calculation of b from equation (1) the numerical value of V,
is required. This value was obtained from the known density of the bulk
polymer d,=0,94 g/cm3.
The constants thus determined are c=0,14 and b=0,44 kbar. With these
constants the Tait-equation well describes the compressional behaviour of
SBR-powder in the pressure range between 2 and 19 kbar and there is a
good agreement between the experimentally determined and the calculated
density/pressure relationship. This is shown in Fig. 1 where the solid curve
is calculated from the Tait-equation with the above constants c and b.
In addition to SBR without carbon-black, HAF-black alone was studied.
The density measurements on HAF-black in Part I of this study' are an
H. Schilling, G. Angerer, and T. S. Ng
example for this. In the pressure range investigated it is not possible to
obtain homogeneous and porosity-free samples. Even at 19 kbar the porosity
in the sample is still about 15 vol.-% (cf. Part 1'). Therefore, it cannot
be justified to determine the bulk modulus -V,.dp/dV on such a sample.
However, Vergnon ' who studied the compressional behaviour of metal
powders at pressures up to 4 kbar, has shown that there exists a linear
relationship between the quantity p.V,/(V,-V) and the pressure p.
where p:
V, : volume of the powder at zero pressure,
V: volurnc of the powder at pressure p.
d o : density of the powder at zero pressure,
d : density of the powder at pressure p,
d,: maximum density which the powder can reach,
k : constant,
p: coefIicient of friction.
We applied this equation to our measurements on HAF-black and found
that the theory is also valid for carbon-black. The relationship between
p' V, /(Vc, - V) and p is indeed linear for both ascending and descending pressure. This is shown in Fig. 3.
Vergnon's'j equation was derived for ascending pressure only and it seems
senseful to limit our further conclusions to this part of the measurements
because do, the density at zero pressure, is not the same for increasing and
decreasing pressure and hence both curves do not coincide.
From the slope of the ascending curve in Fig. 3 it is possible to determine
the maximum density d, which the black can reach. According to VergnonI3
maximum density means density of the closest possible packing of the particles
of a powder. In this finite state there are still voids present which are not
occupied by particle matter.
In our case the numerical value of the slope of the ascending curve is
1,71 and hence d,/(d,-do)=
With d,=0,83 g/cm3 we find for the maximum density d,=2,0g/cm3.
This is in fair agreement with the extrapolated density of 1,97g/cm3 of HAFblack at 19 kbar which should be close to the finite state of packing (cf.
Part I, Fig. 7 ' ) and also with the values of 1,97 to 2,00g/cm3 published
by Voet ' '.
Modulus and Swelling Measurements
P r e s s u r e (kbar)
Fig. 3. Pressure dependence of the quantity p.V, /(V, - V) of HAF-black
Temperature Depmdenw of the Torsional Modulus
In order to obtain further information about the interaction between carbonblack and polymer, the torsional modulus of the various pressed HAF/SBR
powders was measured as function of the temperature. The powders were
first compacted at 20 kbar in the 180 mm x 6 mm x 1 mm pressure cell described
above, and after demoulding their mechanical behaviour was studied in a
testing apparatus (Dunlop-Frank-Torsionsautomat) described by Caspary
which permits to evaluate automatically the temperature dependence of the
torsional modulus in the temperature range between - 150 C and +250 C
according to ASTM D 1053.
The results of these measurements are shown in Fig. 4 for a series of
samples of different HAF : SBR ratios. It is obvious that the torsional modulus
is increasing over three orders of magnitude with increasing carbon-black
content of the samples. The glass transition temperature, however, remains
nearly constant at - 50 'C, regardless of the carbon-blackipolymer ratio. This
is in accordance with observations on sodium-chloride-filled polyurethane
H. Schilling, G. Angerer, and T. S. Ng
rubbers reported by Schwarzl et al.I9. These authors investigated the temperature dependence of the torsional modulus of polyurethanes filled with up
to 70 v01.-Yo of sodium chloride and found a steep increase of the torsional
modulus with increasing NaCl content. They also found that the glass transition
temperature of the polyurethane rubber is not influenced by the incorporation
of NaCI.
Torsional modulus (N.m-*)
Volume r a t i o
79 :21
59 :41
5 3 : 47
Fig. 4. Temperature dependence of the torsional modulus of HAFjSBR powders of
different composition after pressure treatment at 20 kbar.
Modulus temperature measurements are suitable means for characterizing
the different types of cohesion mechanisms in polymers'' and the effect
of crosslinking 2 1 . In polymer-filler systems, however, the situation is more
complex and it is difficult to interpret the modulus temperature behaviour
in terms of polymer-filler interaction and polymer-polymer crosslinkage. Weste. g., did not find a qualitative difference between the modulus
temperature curve of a rubber/carbon-black batch and a rubber/carbon-black
vulcanisate, except for the much lower modulus level of the batch compared
to the crosslinked vulcanisate.
Modulirs and Swrlliny Measuremcwts
The way in which carbon-black aggregates interact with the polymer has
been subject of many theories. Westlinning” proposed adsorption of rubber
molecules on the filler surface which results in immobility and crystallization
of the rubber molecules. The thickness of the adsorbed layer was determined
by X-ray measurements to about 350A at 20’C. As against that thickness
determinations of the layer from modulus measurements which were carried
out by SmitZ3yielded much smaller values in the order of 20
Recently K r a u and
~ ~Medalia2’
developed a theory on the relation between
the voids which are present in each individual carbon-black aggregate and
their influence on the reinforcement mechanism. According to this theory
a certain portion of the elastomer is occluded in the voids of the carbon-black
particles and thus immobilized. The immobilized elastomer acts as part of
the filler rather than as part of the deformable elastomer matrix. The overall
properties of such a carbon-black/polymer system therefore correspond to
a system with higher carbon-black concentration.
Due to the method of preparation in aqueous medium the dispersion
of carbon-black aggregates and latex particles is promoted. However, since
the latex particles have diameters in the order of 200 to 300A the question
arises in which way they interact with the tenfold bigger carbon-black aggregates, especially when there are only a few percent of polymer present in
the whole system. In order to test this HAF/SBR powder of weight composition
75/25 was prepared by addition of HAF-black to a SBR solution and subsequent evaporation of the solvent. From the powder thus obtained samples
were prepared in the usual ways by application of pressure. Contrary to
samples from latex-mixed and spraydried powders of the same composition,
samples prepared from solution had a very limited mechanical strength. This
must be due to the surface covering of the carbon-black aggregates by the
polymer chains which uncoil in solution and are thus capable to surround
the particles evenly in thin layer.
In the example given above 1 g of 75/25 HAF/SBR powder has a carbon-black content
of 0,7Sg with a surface area of about S6m2.On the other hand there are 0,2Sg of
SBR present corresponding to a volume of about 0,27cm3. If this quantity is evenly
distributed over the carbon-black surface a layer of 48 A will result.
This example shows that the irregular covering of the carbon-black surface
by latex particles leads to a stronger bonding between adjacent carbon-black
particles than with comparable solution-prepared powders which are surrounded by a thin polymer shell.
H. Schilling, G. Angerer. and T. S. Ng
The interaction between polymer and carbon-black particles is strongly
promoted by the application of pressure. This is in accordance with the
occlusion concept of K r a u and
~ ~ Medalia".
Under the action of pressure
all carbon-black particles are closely brought together and the polymer which
is free to flow is forced into the voids of the carbon-black aggregates and
thus more or less immobilized.
From Fig. 4 it is possible to extrapolate to a hypothetical elastomer-free
carbon-black body which, however, due to its porosity resembles sinter-metal.
Such a hypothetical carbon-black/air system would correspond to a coherent
body of fused-together carbon-black aggregates with about 55 v01.-% of
air according to measurements published in Part I of this work The torsional
modulus values at 0°C and 1OO'C of Fig. 4 are plotted versus the volume
Torsional m o d u l u s ( N.m'2)
V o l u m e r a t i o HAF: S B R
Fig. 5. Torsional modulus at 0°C and 100°C of HAF/SBR powders ofdifferent composition after pressure treatment at 20 kbar.
Modulus and Swelling Measurements
ratio of HAF:SBR in Fig. 5. When extrapolating the two curves to 100
vol.-O/u of HAF one obtains the torsional moduli for the carbon-black/air
system at
o"c:1,5.109 N/m*
100°C: 5 . l o BN/m2.
Swelling Behaviour and Apparent Crosslink Density
In order to obtain an estimate of the magnitude of the interaction between
the carbon-black and the elastomer molecule aggregates, swelling measurements were carried out on HAF/SBR powders after densification at different
According to the well known theory by Flory and RehnerZ6, FloryZ7,
and extended by Kraus" it is possible to determine quantitatively the number
of crosslinks in vulcanized carbon-black-loaded elastomers from their equilibrium swelling volume. In spite of the fact that our samples were unvulcanized,
they showed limited swelling in toluene, so that we could apply this theory.
For the calculation of the apparent crosslink density of the samples it
was, however, necessary to take into account their porosity. With the assump
tion that the pores will absorb a quantity of solvent which corresponds
exactly to the volume of the pores and the further assumption that the
swelling process in a porous material is basically the same as in a nonporous
one the following relation was applied
v, -vF--vp
vr = ___
v, - v F - v s - 2 v p
where v,:
Volume fraction of polymer in the swollen network,
Volumc of the sample before immersion in solvent,
Volume of filler,
Pore volume,
Volume of solvent which is taken up until equilibrium is reached.
The value of vr thus obtained was introduced into the Flory-Rehner-equations and the apparent crosslink-density was determined with the Hugginsparameter of p0 = 0.2829.
The results of such measurements are shown in Fig. 6 where the apparent
crosslink density of five HAF/SBR powders which were densified in the
cylindrical mould at 19 kbar is plotted versus their composition. The apparent
crosslink density increases exponentially over two orders of magnitude with
H. Schilling, G. Angerer, and T. S. Ng
Crosslink -density ( m o l e s . ~ m - ~ )
100 SBR
Volume ratio HAF:SBR
Fig. 6. Crosslink-density of HAFjSBR powders of different composition after pressure
treatment at 19 kbar.
increasing carbon-black content. It must be mentioned that the accuracy
of the measurements on samples with high carbon-black content is somewhat
limited and accordingly the reproducibility as well.
In order to study the influence of the pressure on the apparent crosslink
density, samples of HAF/SBR powders of composition 75/25 parts by weight
(60/40parts by volume) were prepared by densification at pressures between
1 kbar and 19 kbar and it was found that the apparent crosslink density
moles/cm3 with samples prepared at 1 kbar
increases from about 2 .
to about 9.
moles/cm3 with samples prepared at 19 kbar. Here again
the accuracy of the measurements was limited.
We have deliberately used the term "apparent crosslink density" because
we do not know the true character of the crosslinks in our samples. In
his survey on ,,Bound Rubber" Kraus3' points out that chemical crosslinks
between carbon-black aggregates and polymer molecules as well as between
polymer molecules as such might occur in unvulcanized carbon-black/polymer
systems. Mechanical forces such as shearing forces during milling and mixing
or thermal influences promote this process of bound rubber formation very
Modulus and Swelling Measurements
In our mixing procedure shear forces were applied to the HAF/SBR system
during the high speed stirring of the aqueous HAF/SBR dispersion. During
the subsequent spray-drying process heat was applied as well and both treatments might have caused the formation of "bound rubber". This question
was studied by means of solubility measurements at room temperature for
which the following procedure was chosen:
500mg of powder were mixed with 150ml toluene and were allowed to dissolve for
72 hrs. Then, after shaking, the residual powder was separated by centrifuging at 1O"C,
washed with toluene and weighed after drying at 60°C in vacuo. The weight loss was
expressed as soluble part of the total SBR present in the system.
For the sake of comparison SBR alone was also included in the study.
The results are shown in Table 2.
Table 2.
Solubility of spraydried HAF/SBR powders of different composition in toluene
at room temperature.
HAF/SBR powder
Solubility of SBR (wt.-o/,)
lI o
Carbon-black concentration (vol.-%,)
- .-. . .
.... ..
From this it is obvious that spray-drying causes a high degree of insolubility
of the powders which contain carbon-black whereas SBR alone is hardly
affected by the spraydrying process and remains almost fully soluble.
In order to separate the effect of the thermal treatment in the spray-drying
process from the effect of the mixing process of the HAF-black dispersion
with the SBR latex, fully mixed HAF/SBR dispersions of different composition
were precipitated by means of a 1 : 1 alcohol/acetone mixture. The resulting
precipitates were dried in vacuo at 40°C and their solubilities were determined
as described above. Again SBR alone was included in the study. The results
are shown in Table 3.
It is clearly shown that a strong interaction between carbon-black and
polymer already occurs during the preparation of the dispersion. However,
it is far lower than with the spray-drying process. Again SBR alone is almost
completely soluble.
H. Schilling, G. Angerer, and T. S. Ng
Table 3. Solubility of precipitated HAF/SBR crumb of different composition in toluene
at room temperature.
Carbon-black concentration (vol.-Oh)
Precipitated HAF/SBR crumb
Solubility of SBR (wt.-O/,)
Finally the solubility of the samples prepared from HAF/SBR powders
by compression at 19 kbar was checked by hot extraction with boiling toluene
(bp. 110,8 "C) for 24 hrs. and subsequent drying in vacuo at 70°C. It turned
out that the samples were practically insoluble. This means that the HAF/SBR
structure as obtained by the pressure treatment described remains stable
even in hot solvents and consequently cannot be distinguished by this method
from HAF/SBR-vulcanisate structures which show a similar behaviour under
these conditions.
In general, insolubilization of rubber due to presence of active carbon-blacks
without pressure treatment is a well known phenomenon and Sircar and
Voet3' found out that it is not purely physical in nature, but rather a chemisorp
tive effect. Amongst other factors the degree of unsaturation and the molecular
weight of the polymer are of influence on the magnitude of insolubility of
carbon-black/polymer mixtures.
Repeated Pressure Treatment
In order to study the character of the pressure generated HAF/SBR structure
further, one of the more brittle HAF/SBR samples with a volume portion
of 79% HAF-black was powderized in a mortar and the powder obtained
was once more subjected to a pressure treatment at 19 kbar. A coherent
sample was obtained again, which showed limited swelling in toluene. This
obviously indicates that the HAF/SBR structure generated by high-pressure
treatment is reversible. This is fairly remarkable because chemically crosslinked
systems do not show reversibility; thus, one of the authors32 showed that
pressings of ebonite powder desintegrate completely in benzene.
In order to confirm this behaviour with HAF/SBR-systems, the following
experiment was carried out: A fully compounded HAF/SBR-latex mixture
containing zinc oxide, sulphur and accelerator as vulcanizing ingredients
was prepared according to Table 4.
Modulus und Swelling Measurements
Table 4. Vulcanizing HAF/SBR-latex mixture.
(Dry solids content in parts by weight)
HAF-black, fluffy
Zinc oxide, active
Zinc diethyl dithiocarbamate
This compounded latex was subjected to a thermal treatment (43 hrs. at
85 "C),in order to vulcanize the latex particles in the latex stage. The vulcanized
latex was spraydried to a powder. This powder was pressure-treated at
13 kbar to a pseudo-coherent sample. When this sample was allowed to
swell in toluene it desintegrated completely. In contrast to this, a powder
from the same compounded latex which, however, was not subjected to a
thermal treatment and consequently was not vulcanized, yielded a coherent
sample after pressure treatment; this sample remained intact in toluene and
showed limited swelling.
These experiments characterize distinctly the different structures once
obtained by pressure treatment of SBR/HAF powders and once obtained
by chemical vulcanizing agents and support the idea of a strong polymer-carbon-black interaction rather than that of a strong polymer-polymer interaction
as an explanation for the described phenomena.
It is reasonable to assume that the formation of bound rubber has been
promoted by the application of the high pressure and may have become
the dominant bonding mechanism, leading to pseudo-crosslinked structures
which remain stable even at higher temperature and in hot solvents. From
the present study, however, no conclusion can be drawn about the real nature
of this bound rubber. Thus, it must be left undecided, whether this is a
purely physical absorption phenomenon or whether chemical effects, like
chemisorption or even free radical processes are involved.
H. Schilling, G . Angerer, and T. S. Ng, Angew. Makromol. Chem. 35 (1974) 131
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measurements, powder, effect, blackelastomer, part, modulus, blacksbr, kbar, haf, pressure, swelling, carbon
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